Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/scripts/diff-email-templates.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+make build-templates
+
+if [[ `git status templates/ --porcelain` ]]; then
+  echo "mjml templates have changed; Please compile and include compiled files"
+  git diff templates/  # show diffs as part of error message
+  exit 1
+else
+  echo "PASS"
+fi